Method and apparatus for image processing using adaptive convolution filters

ABSTRACT

A method of spatially filtering an image using an adaptive convolution filter is disclosed. The adaptive convolution filter has two major properties. First, size of the convolution filter adapts to the resolution density of the image being filtered. Second, values of the filter dynamically vary depending on an adaptive value that, in turn, depends on the output resolution.

BACKGROUND

[0001] The present invention relates to the art of processing digitalimages. More particularly, the present invention relates to the art ofimage filtering techniques for visual enhancement of digital prints andphotographs.

[0002] Many techniques exist for processing and filtering signals,especially signals representing two-dimensional (2D) images. As the 2Dimages are increasingly digitized and computerized, it becomes easier toapply many more different filtering techniques than before. Digitizedimages are typically represented by an array (typically but notnecessarily rectangular) of pixels, each pixel represented by a digitalvalue and representing a smallest unit of the digital image.

[0003] Spatial filtering is the filtering of an image in the spatialdomain. That is, the value of each pixel of the image is modified incontextual relationship to neighboring pixels. Typically, spatialfiltering is used to remove noise, to enhance the image, to manipulatethe image into some more visually attractive form, or any combination ofthese. For example, a low pass filter, applied to an image, smoothes theimage by removing finer details such as noise but will also blur imagedetail.

[0004] On the other hand, a high pass filter removes low spatialfrequencies while retaining (“passing”) high frequency information. Thistends to highlight edges and other sharp boundaries. Because of humanvisual perception consideration, an image having enhanced high frequencycomponents appear sharper than an image without enhanced high frequencycomponents. The following is a typical high pass filter in a 3×3(conservative) kernel: −1 −1 −1 −1  9 −1 −1 −1 −1

[0005] Filters are not limited to 3×3 in size, and the values of eachcomponent of the kernel may be different to achieve different filteringeffects. However, because such filters are rectangular in configuration,they tend to amplify the grid-nature, or digital-ness, of the image itis filtering.

[0006] Further, filters of this nature are not necessarily matched ineither scale (or strength) or spatial extent to the natural visual scalethat is optimum for viewing the digital form of the image.

[0007] Consequently, there is a need for a technique and apparatus forfiltering images overcoming these shortcomings of the current art andmatching the digital image in scale and extent to the optimum visualcharacteristics as viewed.

SUMMARY

[0008] The need is met by the present invention. According to one aspectof the present invention, a method of processing an image includesspatially filtering the image using an adaptive convolution filter.

[0009] According to another aspect of the invention, an apparatus forprocessing an image includes a processor and storage connected to theprocessor. The storage includes instructions for the processor to filterthe image using an adaptive convolution filter.

[0010] Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a simplified diagram of a computing apparatus used toimplement one embodiment of the present invention.

DETAILED DESCRIPTION

[0012] As shown in the drawings for purposes of illustration, thepresent invention is embodied in a method of processing an image byspatially filtering the image using an adaptive andcircular-symmetric-approximating convolution filter. Because theconvolution filter is adaptive to the underlying spatial resolution (indots per inch, or dpi) of the image, output resolution density (also indpi), or both, the degree of filtering can be controlled for maximalenhancement according to the needs of teach individual image. In thisdocument, for convenience, phrase “circular filter” or “circularconvolution filter” are used in place of and with the same meaning as“circular-symmetric-approximating filter.”

[0013] Convolution Filter Adaptive to Output Interval

[0014] TABLE 1 below illustrates one embodiment of the adaptiveconvolution filter is illustrated below as a five-by-five (5×5) filter.TABLE 1  0  0 −2A  0 0  0  −9A −14A  −9A  C −2A −14A   C −14A −2A   0 −9A −14A  −9A 0  0  0  −2A  0 0

[0015] where C is 101 plus an adaptive value (designated herein as X forbrevity) and A is (C−1)/100).

[0016] Note that in this example, this convolution filter isconservative (sums to 1) for any value of X. The image-specific value Xmay be any positive value, and may depend upon the density (“outputdensity” or “output interval”) at which the image is to be printed ordisplayed at an output device, in addition to the sharpening need of theimage under application of the filter. For example, for printing at 300dpi, the adaptive value may typically range from ten to 40 or higher.TABLE 2 below illustrates adaptive values typically usable for imagesharpening. TABLE 2 Resolution Adaptive X-value Range Density (dpi)Moderate Strong Aggressive 300 40 20 10 200 80 40 20 150 160 80 40  75320 160 80 Any 10 5 −1

[0017] For instance, if the image is to be printed at 150 dpi, then,using the adaptive value of 160 for moderate filtering effect, TABLE 3Abelow illustrates the filter of TABLE 1 with the adaptive value 160.TABLE 3A 0 0 −2*(C/100) 0 0 0 0  −9*(C/100) 0 0 0  −9*(C/100)−14*(C/100)  −9*(C/100) 0 −9*(C/100) −14*(C/100) 101 + 160 −14*(C/100)−9*(C/100) 0  −9*(C/100) −14*(C/100)  −9*(C/100) 0 0 0  −2*(C/100) 0 0

[0018] Where C=101+160 and A=C/100 resulting in values in TABLE 3Bbelow: TABLE 3B 0 0 −5.2 0 0 0 −23.4 −36.4 −23.4 0 −5.2 −36.4 −261 −36.4−5.2 0 −23.4 −36.4 −23.4 0 0 0 −5.2 0 0

[0019] In one embodiment, the values of the adaptive convolution filtersum to one. However, this is not required to practice the presentinvention, and values that do not sum to one may be used for specialpurposes, such as an accompanying general lightening or darkening theimage. For instance, for a 5×5 filter similar to the filter illustratedby TABLE 1 having C=100+X instead of C=101+X may be used. In this case,the values of the filter sums to a zero. Such filters may be used fordiagnosis of the underlying image structure be effectively contouringthe image.

[0020] Convolution Filter Adaptive to Resolution Density

[0021] The adaptive convolution filter is adaptive in another aspect.That is, the size and the values of the convolution filter are adaptableto the underlying spatial resolution (“resolution interval”) of theimage. This is independent of the output interval. The resolutioninterval is that at which the image is to be printed or viewed.

[0022] For example, the filter illustrated by TABLES 1 or 3B above maybe used for a 300 dpi image. For another, a second, image havingresolution density of 600 dpi, a seven-by-seven (7×7) filter may beused. TABLE 4 illustrates a 7×7 filter that may be used to process a 600dpi image. TABLE 4 0 0 0 −15A 0 0 0 0 −15A −50A −70A −50A −15A 0 0 −50A0 (100 + X)A 0 −50A 0 −15A −70A (100 + X)A 401 + 4X (100 + X)A −70A −15A0 −50A 0 (100 + X)A 0 −50A 0 0 −15A −50A −70A −50A −15A 0 0 0 0 −15A 0 00

[0023] where X is the adaptive value, C=401+4X, and A (C−1)/400. Notethat, in this example, the filter is conservative for any value of x

[0024] To filter images having even higher resolution interval, evenlarger adaptive convolution filters may be used. For instance, for 1200dip images, a nine-by-nine adaptive filter or an even larger filter maybe used.

[0025] Alternatively, the resolution interval of an image may be changed(reduced or increased) for the application of the adaptive convolutionfilter. For instance, a 1200 dpi image may be changed to a 300 dpi imagefor application of the 5×5 filter of TABLE 1. On the other hand, a 150dpi image may be changed to a 300 dpi image for application of the 5×5filter of TABLE 1. Standard extrapolation and interpolation techniquesfor changing the resolution interval of digitized images are known inthe art.

[0026] Computer Implementing Adaptive Convolution Filter

[0027] Referring to FIG. 1, a computing apparatus 10 implementing oneembodiment of the present invention is illustrated. The apparatus 10includes a processor 12, for example a central processing unit connectedto storage 14. The storage 14 includes image files 16, instructions 18for the processor 12, or both. The instructions 18 include instructionsfor the processor 12 to filtering the image 14 using an adaptiveconvolution filter described herein above including, but not limited to,all aspects of the filtering technique and the properties of thefilters.

[0028] The processor 12 and the storage 14 may be connected to eachother via a system bus 20. Output devices such as a monitor 22, aprinter 24, or both may be connected to the system 10 via the system bus20. Capabilities of these output devices 22 and 24 determine displaydensity of the images 14 filtered by the processor 12.

[0029] From the foregoing, it will be appreciated that the presentinvention is novel and offers advantages over the current art. Althougha specific embodiment of the invention is described and illustratedabove, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. For example, othersizes of the adaptive convolution filters may be used to achieve similarresults. The invention is limited by the claims that follow.

What is claimed is:
 1. A method of processing an image, the methodcomprising spatial filtering the image using an adaptive convolutionfilter.
 2. The method recited in claim 1 wherein the adaptiveconvolution filter is adaptive to resolution interval of the image. 3.The method recited in claim 2 wherein the adaptive convolution filter isadaptive to output interval of the image.
 4. The method recited in claim2 wherein the adaptive convolution filter is a five-by-five circularconvolution filter.
 5. The method recited in claim 4 wherein thefive-by-five circular convolution filter filters a 300 dots per inch(dpi) image.
 6. The method recited in claim 5 wherein the five-by-fiveadaptive convolution filter has values 0  0  −2A  0 0 0  −9A −14A  −9A 0−2A  −14A   C −14A −2A  0  −9A −14A  −9A 0 0  0  −2A  0 0

where C is 101 plus an adaptive value and A is (C−1)/100.
 7. The methodrecited in claim 6 wherein the adaptive convolution filter is adaptiveto output interval of the image.
 8. The method recited in claim 7wherein the adaptive value is within a range from one to 40, inclusive.9. The method recited in claim 7 wherein the adaptive value is within arange from ten to 40, inclusive, when the output interval is 300 dotsper inch (dpi).
 10. The method recited in claim 7 wherein the adaptivevalue is within a range from 20 to 80, inclusive, when the outputinterval is 200 dots per inch (dpi).
 11. The method recited in claim 7wherein the adaptive value is within a range from 40 to 160, inclusive,when the output interval is 150 dots per inch (dpi).
 12. The methodrecited in claim 7 wherein the adaptive value is within a range from 80to 320, inclusive, when the output interval is 75 dots per inch (dpi).13. The method recited in claim 7 further comprising a step of changingresolution interval of the image for application of the adaptiveconvolution filter.
 14. The method recited in claim 13 wherein theresolution interval of the image is changed to 300 dpi.
 15. The methodrecited in claim 1 wherein the adaptive convolution filter is adaptiveto output interval of the image.
 16. The method recited in claim 15wherein the adaptive convolution filter is a five-by-five circularconvolution filter.
 17. The method recited in claim 16 wherein thefive-by-five circular convolution filter filters a 300 dots per inch(dpi) image.
 18. The method recited in claim 18 wherein the five-by-fiveadaptive convolution filter has values 0  0  −2A  0 0 0  −9A −14A  −9A C −2A  −14A   C −14A −2A  0  −9A −14A  −9A 0 0  0  −2A  0 0

where C is 101 plus an adaptive value and A is (C−1)/100.
 19. The methodrecited in claim 18 wherein the adaptive convolution filter is adaptiveto output interval of the image.
 20. The method recited in claim 18wherein the adaptive value is within a range from one to 40, inclusive.21. The method recited in claim 18 wherein the adaptive value is withina range from ten to 40, inclusive, when the output interval is 300 dotsper inch (dpi).
 22. The method recited in claim 18 wherein the adaptivevalue is within a range from 20 to 80, inclusive, when the outputinterval is 200 dots per inch (dpi).
 23. The method recited in claim 18wherein the adaptive value is within a range from 40 to 160, inclusive,when the output interval is 150 dots per inch (dpi).
 24. The methodrecited in claim 18 wherein the adaptive value is within a range from 80to 320, inclusive, when the output interval is 75 dots per inch (dpi).25. An apparatus for processing an image, the apparatus comprising: aprocessor; storage connected to the processor, the storage includinginstructions for the processor to filter the image using an adaptiveconvolution filter.
 26. The apparatus recited in claim 25 wherein theadaptive convolution filter is adaptive to resolution interval of theimage.
 27. The apparatus recited in claim 26 wherein the adaptiveconvolution filter is adaptive to output interval of the image.
 28. Theapparatus recited in claim 26 wherein the storage further comprisesinstructions for the processor to change resolution interval of theimage for application of the adaptive convolution filter.
 29. Theapparatus recited in claim 28 wherein the adaptive convolution filter isa five-by-five circular convolution filter.
 30. The apparatus recited inclaim 25 wherein the adaptive convolution filter is adaptive to outputinterval of the image.